CHAPTER VII. 



THE NEURONE OR NERVE-CELL. — ANATOMY OF THE NER- 

 VOUS SYSTEM. — PHYSIOLOGY OF THE NERVOUS SYSTEM ; 

 REFLEXES. 



The neurone. — Just as the anatomical and physiological unit 



of the muscular tissue is the muscle-cell, or as it is often called, 

 the muscle-fibre, so the unit of the ner- 

 vous system is the nerve-cell, or neurone. 

 Thus the structure of the nervous system 

 depends upon the position and relations 

 of the neurones which compose it; and 

 the activity of this system as a whole is 

 the sum of the activities of its neurones. 



Although the neurones or nerve-cells 

 vary considerably in size and in form, 

 there are certain structural characteristics 

 which they all possess in common. The 

 typical neurone consists of a small mass 

 of granular cytoplasm which surrounds 

 a large vesicular nucleus. From this 

 cytoplasm arise processes of varying 

 length and form. The latter are of two 

 kinds. Usually in the first variety (den- 

 drones) the cytoplasm is granular and 

 closely resembles that surrounding the 

 nucleus ; they are usually short, and soon 

 after their origin break up into numerous 

 branches. In the second variety (axones) 

 the processes are not granular, but show 



fine longitudinal striations ; they are often of great length and 



branch only near their termination. 



The nucleus, together with the cytoplasm surrounding it, is 



often called the " cell-body," so we may regard the neurone, or 



74 



Fig. 63. — Diagram of 

 A Neurone. A, axone 

 arising from the cell-body 

 and branching at its ter- 

 mination ; D, dendrones ; 

 C and N, cell-body com- 

 posed of C, cytoplasm, and 

 N, nucleus. 



